Traditional photography relies on exposing a planar portion of the film through a fixed focusing lens (or lenses) so that an object of interest would be precisely focused on the film. The film is disposed perpendicular to an optical axis and located at an image distance si behind the lens that matches an object distance s0 at which the object is located in front of the lens. Especially in macro photography but to a lesser extent with all photography, other objects within the exposed scene but at a distance other than the image distance s0 are out of focus on the film. This basic concept prevalent throughout film photography continued as digital photography was introduced, using a pixilated array in place of the film.
FIG. 1 is a prior art schematic diagram showing a lens 20 focused on an object located at an infinite object distance, so that all light rays 22 incoming to the lens 20 are substantially parallel. The lens is assumed thin for certain assumptions. The focal length f of the lens 20 is that distance behind the lens where parallel light rays entering the lens are focused. The image distance si is that distance behind the lens 20 wherein light through the lens from a specific object distance s0 is focused. Since the object distance s0 in FIG. 1 is far (the light rays incoming to the lens 20 are substantially parallel), then si=f, the image distance is the lens focal length. The sensor 24, which may be film, a digital pixilated surface, a sensing array, etc., is positioned at the image distance si so that the particular object imaged is in focus. This relation is described by the Gaussian (thin) lens formula:
      1    f    =            1              s        0              +                  1                  s          i                    .      For an object located at an infinite distance, s0 is large and 1/s0 becomes vanishingly small, so f≈si. FIG. 2A is similar to FIG. 1, but with the lens 20 moved so that an object at a distance s0<<∞ (e.g., less than about 20′) is instead focused on the sensor 24. The focal length f has not changed between FIGS. 1 and 2A; it is set by the shape and refractive index of the lens 20. Movement of the lens 20 or sensor 24 enables focusing of non-parallel incoming light rays 22 at the sensor 24, and the relation is represented mathematically as
      s    i    =                              s          0                ⁢        f                              s          0                -        f              .  
FIG. 2B is a photograph (such as one taken with the lens arrangement of FIG. 2A) of domino tiles disposed at various distances from the lens, and illustrates depth of field. The image shows three consecutive domino tiles 25, 26, 27, each having a horizontal line and three dots visible above that line. The middle of those three domino tiles 26 is located at the distance s0 of FIG. 2A, and is therefore in sharp focus on the sensor that we view as FIG. 2B. The remaining two 25, 27 of those three domino tiles are in acceptable focus, and are located at respective distances s0-x and s0-x, assuming all domino tiles are equally spaced from one another by a distance x. All other domino tiles are blurred, as they are located more than the distance x from s0. Critical or precise focus may be achieved at only one distance s0 from the lens 20. In FIGS. 1-2B, this distance is the object distance s0.
As is evident in FIG. 2B, there is a range of distances (s0±x) for which focus is acceptably sharp. This zone of acceptable focus is termed depth of field. More technically, the depth of field is the region where the size of the circle of confusion is less than the resolution of the human eye. The circle of confusion is a term well known in the optical and photographic arts, and relates to the fuzziest a point can be and still be considered “in focus”. Increasing depth of field has been the subject of certain improvements to the arrangements of FIGS. 1 and 2A.
One such prior art arrangement to increase depth of field is shown schematically at FIG. 3. In this arrangement, the sensor 24 defines a surface that is tilted and no longer perpendicular to the optical axis 26 defined by the lens 20. An object located nearer the top of FIG. 3 is at a distance s01 from the lens 20. Light rays 22a incoming to the lens 20 are not parallel, is focused precisely at a portion of the sensor 24 at a distance si1 behind the lens 20. Within the same scene and within the same exposure of the sensor is a second object, located nearer the bottom of FIG. 3 and at a distance s0inf from the lens 20. Light rays 22b from this second object are substantially parallel when they enter the lens 20, and are focused precisely on a portion of the sensor 24 that is at a distance siinf behind the lens 20. Because si1 is not equal to siinf, depth of field is increased, i.e. the position of the best focus shifts when moving from the top of FIG. 3 towards the bottom of FIG. 3. The arrangement of FIG. 3 is implemented commercially in the Canon® TS-E 24 mm lens of FIG. 4 allowing to better image objects or scenes which appear “tilted” respect to the camera device. This effect is achieved by arranging a tilt angle between the imaging lens(es) and the sensor plane either by tilting the lens(es) alone or by tilting the sensor itself, as described below.
Another prior art arrangement is described in U.S. Pat. No. 6,783,068, entitled “Large Depth of Field Line Scan Camera”. In this adaptation, a scanning system utilizes a randomly addressable image sensor, which is selectively positioned at the Scheimpflug angle in the image plane in order to detect focused light reflected from an object. Light reflected from the object is focused onto the sensor through an objective lens. Since the sensor is mounted at the Scheimpflug angle, each strip within the depth of field of the object plane has corresponding pixels on the sensor that are in focus.
U.S. Pat. No. 6,567,126 is entitled “Planar Focus Correction” and describes a camera that includes a detector array and an objective lens arranged to direct optical radiation from an object plane onto the detector. The lens defines an optical axis of the camera, and the object plane is at an oblique angle. An optical axis movement device changes the relative orientation of the detector with respect to the lens so that the detector and lens may be moved relatively toward or away from each other along the optical axis and also tilted with respect to each other with at least one degree of freedom. This enables a focus detection device, connected to the detector, to detect when a portion of an image falling on the detector is in focus, holding an in-focus portion of the image in focus until a second portion also comes into focus.
U.S. Pat. No. 6,445,415 is entitled “Increased Depth of Field for Photography” and is described as primarily for electronic cameras. An image is created electronically from a sensor in the camera and is based on a multi photo technique. Several photos are shot with different focused parts of the scene subjects in the respective photo, and a basic image is integrated by contributions from the different images. Calculated image transfers are based on lens- or sensor settings for the respective images.
U.S. Pat. No. 5,282,045 is entitled “Depth of Field Control Apparatus and Image Pickup Apparatus Having the Same Therein”. In this patent, image signals corresponding to a plurality of picture images different in focal point or length position are obtained by a mechanism for changing a focal point or length position to produce a new image signal by composing these image signals through a composition circuit. Motion information of an object is obtained by a circuit for detecting a moving portion in the object to control the image composition by the motion information. The focal point or length position is moved in synchronism with an integer multiple of a vertical scanning period of the television. The image signals corresponding to the plurality of picture images, different in focal point or length position, are obtained within one vertical scanning period determined by the system of the television. The amount of movement of the focal point or length position is controlled in conjunction with a value of a lens aperture of the camera lens. The image composition is controlled by a composition control circuit made up of a circuit for detecting individual powers of image signals corresponding to a plurality of different picture images, a circuit for comparing the detected powers with each other, and a circuit for detecting the position of an edge included in one of the image signals. A control signal for the image composition produced by the power comparison circuit is compensated by the edge position information obtained by the edge detection circuit.
While interesting, each of the above prior art descriptions appear to overlay different planar images of objects within a scene to arrive at a composite having a higher depth of field as compared to any of the individual planar images. However, optical lenses are known to define a Petzval surface or Petzval field curvature as shown in FIG. 5. Briefly, a Petzval surface is a paraboloidal surface at which objects lying along a plane that is perpendicular to the optical axis are precisely focused on the opposed side of the lens 20. Assume a fixed distance of s0 in FIG. 5. Objects at the distance s0 from the optical center 32 in front of the lens 20 are along the curved surface δ0, and absent aberrations are focused precisely at the curved surface 28 behind the lens 20 as defined by the image distance si. These curves δ0 and 28 are typically spherical, but their shape depends upon the lens curvature. Now, flatten the curve δ0 to a plane that is perpendicular to the optical axis 26 at a distance s0, yielding the object plane δ0′. For imaging the scene at the object plane δ0′ at the planar sensor 24, this imposes an additional aberration, because objects along the object plane δ0′ are now precisely focused along the Petzval surface 30, which further diverges from the (spherical) surface 28. It is stipulated that divergence between the planar and curved surfaces of FIG. 5 are exaggerated for clarity of illustration. The planar sensor 24 now images two orders of aberrations, the first due to its non-coincidence with the (spherical) surface 28 defined by the distance si, and the second due to its non-coincidence with the (paraboloidal) Petzval surface 30. The image is focused precisely only at the optical axis, where the sensor 24 is tangent to and co-incident with both curved surfaces 28, 30. Objects imaged off the optical axis are increasingly out of focus. These aberrations are due to field curvature. Using the example of FIG. 2B, the best-focused domino tile 26 may be precisely focused only at the horizontal line of that tile because that line is located along the optical axis, while the top and bottom of that same tile 26 are out of focus due to field curvature, though not particularly noticeable in FIG. 2B. None of the above prior art solutions for increasing depth-of-field are seen to offset field curvature, in either the first or second order. Whereas a four-lens arrangement known in the art may minimize or even eliminate the field curvature illustrated by FIG. 5, such arrangements yield a shallow depth of field, because the object distance at which precise focus is obtained is merely shifted from a curved to a planar surface without additional optical aberration, yet precise focus still lies at only one distance from the lens 20.